The invention relates to a flexible graphite material which has catalytic properties suitable for the preparation of graphite articles useful as components in electrochemical devices such as fuel cells and double-layer capacitors.
The production of efficient, low-cost electrochemical devices like fuel cells is dependent upon the availability of suitable construction materials.
Electrochemical devices like fuel cells are foreseen by some as necessary to the commercial realization of low-emission vehicles as well as a number of stationary power needs. Fuel cells cleanly and efficiently convert suitable fuels to electrical energy. Their unique advantages make them promising for many power applications. In all cases, a balance must be struck between weight and performance, and it would be desirable to adjust manufacturing procedures of current construction materials to assure that both concerns are effectively addressed to provide a net improvement in the operation and/or economy of these devices. Flexible graphite sheet material has proved itself to be a highly effective construction material for fuel cell components and other applications.
Among the fuel cells where components of improved flexible graphite foil could be of advantage are ion exchange membrane fuel cells. Material selection and processing often favors flexible graphite foil due to its overall favorable combination of physical and electrical properties. Proton exchange membrane (PEM) fuel cells are of particular interest. Cells of this type produce electricity through the chemical reaction of hydrogen with oxygen from the air. Within the fuel cell, electrodes denoted as anode and cathode, surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly (or MEA). In some cells, the electrode component will also function as a gas diffusion layer (GDL). A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or other platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. It would be desirable to improve the catalytic activity of the electrode surfaces.
In operation of a PEM cell, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which join with the protons migrating through the proton exchange membrane and the electrons through the circuit. The result is the generation of current and the formation of water. Since the membrane is an electrical insulator, the electrons cannot directly cross the membrane, but seek the least resistance and travel through an external circuit which utilizes the electricity before the electrons join the protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen can be removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
There remains a need in the art for improved materials of construction for fuel cell components which can improve needed catalytic properties.
Accordingly, it is an object of the invention to provide a catalyzed expanded graphite material having an array of desirable properties, including electrochemical catalytic activity useful in making electrochemical fuel cell components.
It is another object of the invention to provide materials and methods for imparting catalytic properties to flexible graphite materials, making them useful starting materials for forming a variety of shaped articles useful as components in fuel cells.
These and other objects are accomplished by the present invention, which provides a material useful as a substrate for forming electrochemical fuel cell elements and methods for preparing materials of this type.
The material of the invention is useful as a substrate for preparing articles such as an embossed or unembossed flexible graphite sheet, the material comprising: a compressed sheet of graphite comprised of intercalated graphite particles having an electrochemical fuel cell catalyst included therein.
According to the process of the invention, flexible graphite sheet is intercalated to an extent necessary to include an electrochemical fuel cell catalyst within graphite particles and then shaping the resulting sheet into any of a variety of shapes by any of a variety of processes.
Many preferred and alternative aspects of the invention are described below.